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It's very difficult to take a purportedly scientific paper which opens with the following phrase seriously.

The health effects of a Roundup-tolerant genetically modified maize (from 11% in the diet), cultivated with or without Roundup, and Roundup alone (from 0.1 ppb in water), were studied 2 years in rats. In females, all treated groups died 2–3 times more than controls, and more rapidly.

Given that most mortal beings only have to go through dying the once it would seem that we are discussing the prevalence of reincarnation in rats. One can imagine interest in such research from various religious groupings but it would be difficult to describe the process as science. However, yes, that is a cheap shot: mocking non-native English speakers for their difficulties with the English language is such. What they mean is that more of the female rats on the GM diet died and died younger than the control group. Which is at least the beginnings of a piece of scientific research.

For yes indeed we really would like to know whether Monsanto's Roundup Ready corn (maize to Europeans) does cause health problems. We don't think it does for several reasons. The multiple studies that have been done before looking at this very question for example. The fact that hundreds of millions of animals have been fed the stuff for years without anyone noticing anything odd about said animals. We've even got a nice natural experiment going on. Those humans in the Americas (North, South and Central) have been eating GM corn in vast quantities for a number of years now. Those humans in Europe have not. Again, we have not noted any difference in disease prevalence among the two groups that cannot be and is not explained by other factors.

But this paper claims to take us into new areas: instead of studying the rats only for 90 days as with earlier studies they studied them for their natural lifespan, around two years. They also claim to have found very different results from everyone else. Taken in the abstract this is fine: that's what science is for. Finding out things that we didn't know before.

Today, researchers led by Gilles-Eric Séralini at the University of Caen in France announced evidence for a raft of health problems in rats fed maize that has been modified to be resistant to the herbicide Roundup. They also found similar health problems in rats fed the herbicide itself.

The rodents experienced hormone imbalances and more and bigger breast tumours, earlier in life, than rats fed a non-GM diet, the researchers claim. The GM- or pesticide-fed rats also died earlier.

This kind of GM maize accounts for more than half the US crop, yet the French team says this is the first time it has been tested for toxicity throughout a rat's lifespan (Food and Chemical Toxicology, DOI: 10.1016/j.fct.2012.08.005).

If this is true then we definitely want to know about it. Yes, we really do, whatever it might do to Monsanto's share price or business. In fact especially given what it might do to that business.

The question is though, is it true? The politest thing we can say about the research so far is that they haven't managed to prove that it is true, no. Scientists working in the area have been, how shall we put this, less than complimentary about some of the methods used:

Prof Tom Sanders, head of the nutritional sciences research division, King's College London, warned the type of rat used was very prone to mammary tumours, particularly when food intake was not restricted. And Dr Wendy Harwood, senior scientist, John Innes Centre, said: "The full data set has not been made available, but the findings do not contradict previous findings that genetic modification itself is a neutral technology, with no inherent health or environmental risks.

"Without access to the full data, we can only say that these results cannot be interpreted as showing that GM technology itself is dangerous. However they do indicate possible concerns over long-term exposure to Roundup that require further study."

Tom Sanders, head of the nutritional sciences research division at King's College London, noted that Seralini's team had not provided any data on how much the rats were given to eat, or what their growth rates were.

"This strain of rat is very prone to mammary tumors particularly when food intake is not restricted," he said. "The statistical methods are unconventional ... and it would appear the authors have gone on a statistical fishing trip."

"The most evocative part of the paper is those pictures of tumorigenesis," said Prof Maurice Moloney from Rothamsted Research, where much UK GM study is undertaken.

"They give the impression that this never happens in controls. I'd be surprised if it didn't, but that ought to be explicitly demonstrated, and if there was a control that ended up showing similar kinds of tumorigenesis then a picture of that rat should be shown as well, just so we can see if there are any qualitative differences between them."

There are more detailed complaints as well. Here's one explanation of why the graphics in the paper are terribly misleading.

But the real killer criticisms come in the statistics they have used. Or rather, the standard statistical techniques that they have not used. You can see discussions of them here and here. The problem being that given the design of the experiment, the number of rats used and then the way the information has been presented to us we simply do now know, and cannot work out, whether this is a result of the effects of GM corn, Roundup or pure blind chance. And that is the point of the various statistical tests, to attempt to exclude pure blind chance as a cause of what is being observed.